Attempts have been made to employ the recombinant DNA technology for cloning murine RNA tumor viruses in the lambda-E. coli system. The viruses interested include HaMuSV and MoMuLV. To facilitate cloning experiments new lambda vectors have been constructed for cloning DNA fragments generated by Sal I enzyme ranging from 4 kb to 18 kb. The viral DNA substrates were obtained from replicating DNA intermediates present in the infected cells including circular and linear forms of genomes. These DNA molecules were isolated, enriched and tailored if necessary before cloning. Using this approach recombinant lambda phages containing portions and/or a full-length copy of the viral genome would be obtained. Further experiments are underway to construct the deletion mutants at specific sites by shortening the cloned HaMuSV DNA and propagate the resulting DNA molecules in lambda-E. coli system. The biologic properties of these mutants will be examined and their functional defects correlated with respect to the deleted sequences.